Practical measurements of water stable isotopes in tree stems and soils using conservative water vapor storage

The interest of inferring plant water uptake patterns grew in recent years and studies have shown the use of in-situ measurements based on laser absorption spectroscopy making plant water stable isotope datasets available on-site and in real-time. However useful, in-situ systems are limited to sites with power supply and require constant care.

We tested, first in the lab and then in the field, a method for equilibrating, collecting, storing, and finally analysing water vapour for its isotopic composition. We used a vapour storage vial system (VSVS) that relies on in-situ sampling, and measuring the samples in a laboratory. We tested the utility of the sampling method and the reliability of the VSVS to faithfully store the isotopic composition of its content by sampling a range of water vapour of known isotopic compositions and measuring the isotopic signature after the storage period. Samples for the field trial were taken in a boreal forest.

We were able to prove the utility of the sampling method within defined uncertainties (0.6 to 4.4‰ δ²H; 0.6 to 0.8‰ δ¹⁸O) for natural abundance. We detected a small change in the isotopic composition of the sample after a longer storage period, which was consistently greater for oxygen but correctable by linear models.

Our method combines the best of two worlds: sampling in-situ in high spatial or temporal resolution while measuring in the laboratory, giving the community a tool that is not only cost-efficient but also easy to use while all components are commercially available.